At a mainframe computer or other data processing center of substantial size there are usually one or more data cables, converging on the data processing center from a plurality of peripheral stations. A typical cable may contain sixteen data lines, each data line including two data conductors and a return conductor; the same cable usually includes two ground lines, so that a conventional fifty pin connector serves to connect the cable to the computer. Of course, other cable sizes employing different connectors (e.g., twenty-five or one hundred pins) are also employed in essentially the same way. In any such data system there is a preselected voltage range for transmission of data on the data conductors, usually balanced with respect to the return conductor; a typical range is fifty volts, twenty-five volts of each polarity relative to ground.
Any voltage outside the preselected data voltage range, on any of the conductors, may cause physical damage to the computer, to the peripheral stations, or to the cable, in addition to interfering with the data transmission. This problem applies to very brief voltage transients as well as to voltage surges of appreciable duration; any and all overvoltage surges and transients may be quite deleterious. Thus, the cable, or separate data lines, and particularly the computer or other data processing equipment, need protection against overvoltage surges of any origin. That is the aim of the present invention.